1. Field of the Invention
The present invention relates generally to thermistor elements, and more particularly, to a thermistor element which is suitably used as a surface mounted type tip component and is subject to small variations in resistance value and B-value.
2. Description of the Prior Art
One example of NTC (negative temperature coefficient) thermistor elements conventionally used as a surface mounted type tip component is shown in FIG. 2. An NTC thermistor element 1 has a structure in which electrodes 3 and 4 are formed on both end surfaces of a single plate type thermistor body 2. Used as the thermistor body 2 is a ceramic sintered body obtained by cutting to a predetermined size a ceramic wafer obtained by slicing a ceramic sintered body block.
The above described NTC thermistor element 1 has been conventionally fabricated in the following manner. More specifically, ceramic powder for forming the thermistor body 2 is first calcined, to obtain a calcined raw material. A binder is then mixed with the calcined raw material, and a mixed raw material obtained is granulated. The granulated raw material is formed to a predetermined size, to obtain a formed body. The formed body is sintered and is sliced in the direction of thickness, to obtain a ceramic wafer having a thickness corresponding to the thickness of the element. The ceramic wafer is annealed at temperatures of 900 to 1100.degree. C. and then, is cut to a predetermined size by a dicing saw and is barrel polished and then, outer electrodes 3 and 4 are formed on both end surfaces thereof. The outer electrodes 3 and 4 are formed by applying conductive pastes such as Ag pastes and baking the same at a predetermined temperature for approximately ten minutes.
The conventional tip type NTC thermistor element obtained in the above described manner has not been widely applied at the present time. The reason for this is that the variation in resistance value of the NTC thermistor element 1 obtained is large, i.e., tens of percent, and the variation in B-value is also very large, i.e., approximately 1.0 to 2.0 percent. Consequently, it is desired that the variation in resistance value and the variation in B-value are reduced.
TABLE 1 ______________________________________ RESISTANCE B-VALUE R25.degree. C. R3CV B.sub.25/50 B3CV CHIP SIZE (k .OMEGA.) (%) (k) (%) ______________________________________ 3.2 .times. 1.6 .times. 1.0 15.125 13.2 3502 1.8 2.0 .times. 1.25 .times. 1.0 11.536 15.2 3489 1.0 ______________________________________
On the other hand, in the NTC thermistor element 1 shown in FIG. 2, the resistance value thereof is adjusted by changing the thickness of the ceramic body 2. More specifically, when the resistance value of the NTC thermistor element 1 deviates from a desired resistance value, the resistance value is adjusted by decreasing the thickness of the thermistor body 2 and specifically, the thickness of the above described ceramic wafer by polishing or changing the thickness to which the ceramic sintered body block is sliced to change the thickness of the ceramic wafer. Therefore, the variation in thickness is significantly large between manufacturing lots, so that NTC thermistor elements are forced to greatly vary in thickness although they have the same resistance value. As a result, the NTC thermistor element having a small thickness has the disadvantage of being, for example, chipped or cracked in the actual use.
Furthermore, NTC thermistor elements having a series of resistance values are respectively constructed using different types of materials. Consequently, if an attempt is made to make the thicknesses of the NTC thermistor elements having a series of resistance values constant, one type of material provides only one type of NTC thermistor element having a definite resistance value, so that a large number of types of materials are required. In order to avoid this, a series of types of NTC thermistor elements each having a definite resistance value are made from one type of material by changing the thicknesses of the elements. Therefore, the NTC thermistor elements having a series of resistance values greatly vary in thickness from 0.5 to 1.3 mm
Additionally, the conventional NTC thermistor element 1 has the disadvantage of easily varying in characteristics with time and having insufficient life characteristics because the outer electrodes 3 and 4 are exposed to the surfaces.
Moreover, the variation in resistance value of the conventional NTC thermistor element 1 is significantly affected in construction by the variation in size of the thermistor body 2 and the variation in size between the outer electrodes. Accordingly, significantly high precision is required for the NTC thermistor element to take a desired resistance value. On the other hand, a PTC (positive temperature coefficient) thermistor element has the same disadvantages as those of the NTC thermistor element.